The present invention relates to an improved process and system for producing alcohol by fermentation.
In a conventional plant for the production of alcohol by fermentation, the "beer" from the fermenters is sent first to a distillation column or columns. Here the alcohol is stripped from the beer and is concentrated to remove most of the water. The energy for stripping and concentrating the alcohol comes from steam injected at the bottom of the column or columns. The spent beer from the distillation system is called "distillery slop" and has value as an animal feed if much of its water content is removed. Otherwise, its disposal could be a serious problem. Concentrating this slop to produce animal feed consequently has long been an industry practice, usually by use of multiple-effect evaporators.
The energy requirement for producing alcohol with the conventional process and system has been so high that the cost of producing the alcohol has been about the same as its fuel value. More specifically, the distillation column in the conventional plant has two basic sections, the beer still and the rectifier. The beer still is located below the point of beer addition, and steam is admitted at the bottom to strip out the alcohol. Alcohol losses with the slop can be minimized by increasing steam input or providing more plates or trays in the beer still, but these expedients increase either capital expenditures or operating costs and therefore must be balanced against the increased recovery of alcohol.
Above the beer still is the rectifier section, which serves to separate alcohol from water, with the alcohol leaving at the top and the water, from both the beer and the added steam, leaving as a diluent of the slops at the bottom. The total distillation section may have on the order of 50 plates and use on the order of 20 pounds of steam per gallon of 190 proof alcohol produced. The large number of plates requires such a tall vessel height that the column is frequently split into two sections, one section being the beer still with about 20 plates and the other the rectifier with about 30 plates.
The slop withdrawn from the bottom of the beer still is then concentrated, usually by use of multiple effect evaporators, which requires still more energy. As indicated above, the amount of energy necessary for concentrating the alcohol and distillery slop presently has about the same value as the alcohol that is produced.
A solution to these problems is disclosed in Ser. No. 206,170, the parent application of the present application, which discloses integrating the alcohol distillation and slop concentration in a highly efficient process and system employing a vapor recompression evaporator for slop concentration by employing evaporator vapors as the stripping steam for removing alcohol from the beer, by using the condensing side of the evaporator heating surfaces as a means of enriching the alcohol, and by using the evaporator vents as the source of alcohol and of heat for bringing the alcohol up to desired concentration. According to the process and system of the previous disclosure, all of the energy savings were in terms of thermal (steam) energy, with little or no change in the amount of mechanical (electrical) energy needed to drive the compressor.
On the other hand, another desirable objective in industrial plants is to achieve cogeneration of thermal and mechanical energy requirements, which places constraints on the relative amounts of thermal and mechanical energy used. A study by Raphael Katzen Associates for the Department of Energy (HCP/J6639-01) includes a partial attempt to achieve this objective in an ethanol plant. High pressure steam is generated in a boiler using coal as a fuel. This steam is expanded through a turbine to drive the evaporator's vapor compressor. The low pressure turbine exhaust steam then provides most of the thermal energy needs of the plant, the largest ones being in the distillation and mashing sections. Finally, the stack gas of the boiler is used to dry the spent grain and concentrated slop from the evaporator. Although this system reduces the mechanical energy required, it is far less efficient than the system disclosed in my previous application.